1. Field of the Invention
This invention relates to an image processing method and apparatus which encodes image data and performs processing using the encoded image data.
2. Description of the Related Art
Heretofore, in an image communication terminal having an external storage unit having a relatively large capacity, received image data have been stored in the external storage unit together with additional information represented by character codes, such as the name of an image, the date and the like. The stored data have been read from the external storage unit whenever necessary, and the read data have been displayed on the picture surface of a display unit, or output using a printer. If a large number of received image data are present, a summary of additional information, such as the names of images and the like, of the received image data has been output on the display unit or on paper to confirm the received image data.
In the above-described prior art, however, in order to grasp the entire contents of the received large number of images, only the summary of additional information represented by character codes can be utilized, and it has been impossible to confirm the images using image information. Hence, in order to confirm the contents of the images, it has been necessary to output desired image data every time on the display unit or on paper, which can take a long time.
It has been proposed in U.S. patent application Ser. No. 07/355,116, assigned in common with the present application, to use a technique wherein a reduced image of an original image is stored as an index image, and a desired original image is selected from among a plurality of index images.
In the above-described technique, however, there is room for improvement in the efficiency of image storage since the amount of data to be stored is still large.
As for encoding of image data, in a facsimile apparatus which is a typical example of a conventional still-picture communication apparatus, a method has been adopted wherein an image is sequentially scanned in the direction of the raster, encoded and transmitted. In this method, in order to grasp the entire contents of the image, it is necessary to transmit encoded data of the entire image. Hence, a long transmission time is needed, and it has been difficult to adapt the method to image communication services, such as an image data base service, videotex and the like.
In order to promptly grasp the entire contents of an image, progressive encoding has been considered. FIG. 30 shows an example of conventional progressive encoding.
In FIG. 30, frame memories 501-504 store images reduced to 1, 1/2, 1/4 and 1/8 in size, respectively. Reduction units 505-507 provide images reduced to 1/2, 1/4 and 1/8, respectively. Encoders 508-511 encode images reduced to 1/8, 1/4 and 1/2 and 1, respectively.
The reduction unit 505 reduces an image from the frame memory 501 by, for example, a method to subsample the image to 1/2 in both the main scanning and subscanning directions to provide a 1/2-sized image, and stores the reduced image in the frame memory 502. The 1/2-sized image is further reduced by the reduction unit 506 to provide a 1/4-sized image, and the reduced image is stored in the frame memory 503. Similarly, a 1/8-sized low-resolution image is provided by the reduction unit 507, and the reduced image is stored in the frame memory 504.
By sequentially encoding and transmitting the stored images from lower-resolution images, rough entire images can be promptly grasped. In the FIG. 30 example, an image is reduced to 1/2, 1/4 and 1/8 in both the main scannning and subscanning directions, encoding is performed in the order of 1/8, 1/4, 1/2 and 1 (an original-sized image), and transmission is performed also in this latter order. In encoding the 1/8 image, the 1/8 image stored in the frame memory 504 is sequentially scanned, and a target picture element to be encoded is referred to surrounding picture elements by the encoder 508 to perform entropy coding, such as arithmetic encoding or the like. As for the 1/4 image, the encoder 509 performs encoding by referring picture elements surrounding the target picture element from the frame memory 503 to picture elements surrounding the 1/8 image from the frame memory 504. The encoding efficiency is thereby increased. Similarly, the encoders 510 and 511 perform encoding by referring the 1/2 image stored in the frame memory 502 and the original-sized image stored in the frame memory 501 to the 1/4 image stored in the frame memory 503 and the 1/2 image stored in the frame memory 502, respectively.
In such a progressive encoding method, in order to maintain information of low-resolution images, various reduction methods are used. For example, a method wherein, after passing through a low-pass filter, an image is again subjected to binary coding, and subsampling is performed, a method to prevent the disappearance of a specific image by providing exception processing in filter operation, and other methods have been considered.
As described above, in the progressive encoding method, by sequentially encoding and transmitting reduced images from lower-resolution images, it is possible to promptly transmit the entire image.
In this method, however, in order to maintain image information of low-resolution images, reduction is repeated several times. Since the order of encoding and transmitting images is the reverse of the order of reduction processing, images of all stages including an original image must be stored in memories. As a result, this method has the disadvantages that the size of the apparatus becomes large and processing becomes complicated.
Furthermore, the progressive encoding method is not suitable for a method to sequentially encode and transmit an image, for example in a facsimile, for the following reasons.
In facsimile communication, since paper is usually used as a recording medium, low-resolution images in intermediate stages are unnecessary in any event. Moreover, since an inexpensive and polular terminals are most often used, progressive encoding which requires frame memories is unsuitable for facsimile communication.
When progressive encoding is applied to an image data base or the like, it is possible to consider adopting the lowest-resolution image as an icon to be used for data base retrieval. In this case, images other than an original image are sequentially decoded to be mainly used only as intermediate media for obtaining the original image. Hence, this approach has the disadvantages that unnecessary images must be stored in the data base, and a long time is needed for a decoding operation.
A technique has been disclosed in U.S. Pat. No. 4,823,193 wherein a part of the images stored in memories is recorded at the transmission side together with information of the communication partner.
In this technique, however, it is impossible to confirm whether or not the information stored in the memories has the content that the operator actually wants to transmit.
That is, for example, when an original comprising plural sheets is read by an autofeeder and stored in memories, a case may arise wherein the feeder feeds two or more sheets at once, and information in which part of the original to be transmitted is lacking is stored within the memories.
In such a case, it has been impossible to recognize which sheet of the original is lacking in a simple manner.
Furthermore, at the reception side, if a large amount of data has been received, a long time is needed for normally reproducing all received images, and also a large amount of paper is needed.
Particularly when only part of the received data is needed, there is the possibility that time and recording paper are wasted.